Abstract
Southern Hemisphere analyses from the surface to 2 mb and from 20 to 80°S for the period May-September 1979 have been used to study the structure of traveling planetary waves. Space-time cross- spectral analysis of the height field has been employed to define the amplitude and phase for both the eastward and westward moving components of particular combinations of zonal wavenumber and frequency band. Latitude-height contour plots of power, phase and coherence squared show that westward moving waves have structure characteristic of barotropic external modes and are coherent across a broad range of latitudes and from the surface to 2 mb, the highest level analyzed. Eastward-moving waves, on the other hand, have more rapid phase variations, especially in the troposphere, and appear more baroclinic. The tropospheric structure of the moving components of wavenumbers 1–4 is as one would expect for baroclinically unstable modes of the Charney type. Wavenumbers 1 and 2 both have double amplitude maxima in the troposphere, separated by ∼20° of latitude. These amplitude maxima are coherent with each other and are about 180° out of phase. The variances of the eastward components of wavenumbers 1 and 2 increase rapidly with altitude in the stratosphere, but the variance in the upper stratosphere is not coherent with that in the troposphere. To explain these observations it is suggested that two linearly independent eastward moving modes are present simultaneously in the Southern Hemisphere, and that these modes are manifestations of the baroclinic instability of the zonal mean flow. One of the modes dominates the variance in the troposphere (Charney mode) and the other dominates the variance in the stratosphere (Green mode).
